1. Field of the Invention
The present invention relates generally to the method of manufacturing semiconductor device, more specially to the method of forming dielectric film of triple polytype flash electrically erasable programmable read only memory (hereinafter, refer to as EEPROM) cells displaying a multiple layer structure.
2. Description of Prior Art
A conventional method of forming triple poly type of EEPROM cells can be explained with reference to FIG. 1.
First, a gate oxide film (which is not shown) is formed on the active region of a semiconductor substrate (10), which has been divided into active and field regions, and the a first polysilicon film for making a floating gate is formed on the gate oxide film. The first polysilicon film is to remain on the active region by patterning it through photolithography. Then ONO (oxide/nitride/oxide, 12,13,14) forms as a first dielectric film and a second polysilicon film forms thereon subsequently, and the control gate (15) forms by patterning the second polysilicon film through photolithography. The first dielectric films (12,13,14) and the first polysilicon film are etched sequentially through a so-call self-aligned etch so that a structure consisting of the floating gate (11), the first dielectric film (12,13,14) and the control gate (15) forms on the semiconductor substrate. Thereafter, the second dielectric film (16) forms outside the entire structure through an oxidation process, and a third polysilicon film (17) forms in order to create a select gate outside the entire structure.
In the conventional method stated above, ONO film forms as first dielectric film (12,13,14) as the first polysilicon film undergoes through a thermal oxidation process. However, in order to forbid the overgrowth of the dielectric film because of the reduction of capacitance which occurs when the dielectric film grows excessively thick, impurities are not fully implanted to the first polysilicon film. Accordingly, when the second dielectric film (16) forms through the oxidation of polysilicon film after the first dielectric film and the first polysilicon film are etched through the self-aligned etch, then the oxide film near the first polysilicon film do not grow thick enough to the extent necessary and changes into an unusual type of oxide film (so-called, Dog ear, de). FIGS. 2 and 3 show TEM (tunneling electron microscope) of a sample formed in accordance with conventional method as mentioned above, and also SEM (scanning electron microscope), which illustrates the section of EEPROM cells formed in a conventional semiconductor manufacturing process. These Figures show that the unusual type of oxidation(de), which is made from the first polysilicon film, forms at the side walls parts of the floating gate (11).
Also, after the second polysilicon film has been etched through the conventional method, overetching should be implemented to get rid of nitride portion existing in the first dielectric film consisting of ONO film in the field region. This causes the loss of oxide film in the field region, which does not have the first polysilicon film during the self-aligned etch, thereby a defect results since the thickness of the oxide film decreases.